AcknowledgmentsFirst and foremost, I would like to thank my parents, without whose tireless support and encouragement none of my achievements, academic or otherwise, would have been possible.In particular, I am grateful to my father, Adam, for instilling in me a deep enmity for ignorance, a lasting appreciation for the beauty of mathematics, and an uncompromising respect for the rigor of science. I am grateful to my mother, Rena, for convincing me that I was capable of achieving any goal I set and for instilling in me a wellspring of self-confidence which sustains me to this day.I am also thankful to my brothers for their guidance, advice, and mentorship as well as for the often contrasting examples they set for me. I am grateful to Yoseif for convincing me that computers are fun, for showing me how to use them, and for introducing me to the art of computer programming by teaching me BASIC when I was seven years old. I am grateful to Daniel for setting an example of stratospheric success, for treating me like a friend rather than a little brother, and for offering a bottomless well of empathy for all the trials of graduate school and life in general. In reinforcement learning, an autonomous agent seeks an effective control policy for tackling a sequential decision task. Unlike in supervised learning, the agent never sees examples of correct or incorrect behavior but receives only a reward signal as feedback. One limitation of current methods is that they typically require a human to manually design a representation for the solution (e.g. the internal structure of a neural network). Since poor design choices can lead to grossly suboptimal policies, agents that automatically adapt their own representations have the potential to dramatically improve performance. This thesis introduces two novel approaches for automatically discovering high-performing representations.The first approach synthesizes temporal difference methods, the traditional approach to reinforcement learning, with evolutionary methods, which can learn representations for vi a broad class of optimization problems. This synthesis is accomplished via 1) on-line evolutionary computation, which customizes evolutionary methods to the on-line nature of most reinforcement learning problems, and 2) evolutionary function approximation, which evolves representations for the value function approximators that are critical to the temporal difference approach.The second approach, called adaptive tile coding, automatically learns representations based on tile codings, which form piecewise-constant approximations of value functions. It begins with coarse representations and gradually refines them during learning, analyzing the current policy and value function to deduce the best refinements.This thesis also introduces a novel method for devising input representations. In particular, it presents a way to find a minimal set of features sufficient to describe the agent's current state, a challenge known as the feature selection problem. The technique, called Feature ...